Web server on SAN vs SSD

Question

We are running 3 PHP web servers with the web directory (including the php files to be ran locally) pulled from a their own local SSD drive. We are deciding wheter to install a SAN to server our web directory from. The main reason for the switch to the SAN is to keep the files in sync and because the SAN is much more stable.

Does anyone have experience with the latency and throughput of a SAN vs a local SSD? Or what kind of performance loss/gain we will get.

Answer 1

Unless your web-server is much busier or larger than is typical, the performance difference shouldn't be significant. The few areas where an SSD may outperform is if you're serving tens to hundreds of millions of files through the web and they're all on the same file-system. In that case the low latency of an SSD will shine through.

In the more typical "tens to hundreds of thousands" serving case the increased latency of the SAN should be markedly less than the latency already incurred by serving over the WAN. If you are concerned about latency, make sure your SAN contains lots of spindles instead of lots of space.

Answer 2

You're not clear but I think you've got the SSD in one server, sharing it to the others via some form of network share - is that right?

If that's the case then unless it's one hell of a SAN the SSD will be significantly faster in terms of both latency and throughput for the server that contains the SSD. The other machines getting at this data over the network will be limited by that network link and the sharing capabilities of the machine with the SSD - even with the fastest network setup this will be much slower than the SSD and is likely to be much slower than a SAN.

So if my assumption about your usage is right then you'll have one very quick server and two much slower servers (if they do perform a lot of disk work anyway, it's irrelevant if not).

If you do a lot of disk work and want consistent performance then moving to a SAN will help with that, no single server is likely to have quicker disk access than another - and it's likely to be perfectly adequate for most needs too.

Answer 3

SSD might help with log files on a very busy system. but is probably a bad use of resources.

Depending on the size and access distribution of your web site, you may find that most of the data is served from buffer cache in memory. In this case disk access statistics are relatively insignificant.